F21 indicates a fault with the main control board / motor circuit.
Try resetting the code first.
Turn the knob to off.
Turn the knob to the 6 o'clock position.
Press and hold the spin button.
Whilst holding it, turn the knob to the 7 o'clock position.
Continue holding for 5 seconds then let go of the button.
Turn the knob to a programme.
If all is well the F21 should have gone.
Try the machine again.
If F21 remains, try the reset again.
If after all this the code remains you'll need to check the motor, wiring and the connections to the main control board.
After this it's a case of replacing parts I'm afraid.
Hope this helps
Let me know how it goes or if you need more help